DuQuoin is in Perry County, named for Oliver Hazard Perry, the winner of the battle of Lake Erie in the War of 1812. All of the surrounding counties are named for slaveholders.
A lot of the initial settlers of southern Illinois came from Southern states. Slavery was banned there by the Northwest Ordinance but after Illinois became a state a proposal to allow slavery lost only by a 4-3 margin among the voters. In Mark Twain's Huckleberry Finn, set in a town on the Mississippi River, the runaway slave Jim can't just cross the river into Illinois--he would not have been safe there. Of course that is a novel but it reflects Twain's memories of the antebellum period in that area.
While not a slave state per se, the second Illinois Constitution (1848) contained specific language banning blacks from settling in the state.
My guess is that Mark Twain may have been aware of this policy when writing Huckleberry Finn. In fact, the exclusionary policy was mentioned by US Senator Stephen A. Douglas during the Lincoln-Douglas debates of 1858.
This 1848 constitution was replaced in 1870. Currently, Illinois is governed by its fourth constitution (1970).